Reinforced micro-mechanical part

ABSTRACT

The micro-mechanical part, for example a horological movement part, includes a silicon core ( 1 ) all or part of the surface ( 3 ) of which is coated with a thick amorphous material ( 2 ). This material is preferably silicon dioxide and has a thickness which is five times greater than the thickness of native silicon dioxide.

This is a National Phase Application in the United States ofInternational Patent Application PCT/EP 2006/005959 field Jun. 21, 2006,which claims priority on European Patent Application No. 05013912.0,filed Jun. 28, 2005. The entire disclosures of the above patentapplications are hereby incorporated by reference.

TECHNICAL FIELD

The present invention concerns a micro-mechanical part made of silicon,said part having been treated in order to give improved mechanicalproperties. It is for example, but in a non limitative manner, amicro-mechanical part for a horological mechanical movement, i.e. eithera part having an active function for example for transmitting and/ortransforming an energy to drive hands in order to give a time indicationin connection with a dial, or a passive part for example for positioningwheel sets.

BACKGROUND OF THE INVENTION

Silicon is a material which is used more and more often in themanufacture of mechanical parts and in particular of micro-mechanicalparts, both “captive” parts i.e. parts which stay connected to asubstrate on which they have been etched, or “free” parts such as partsbelonging to the kinematic chain of a horological movement.

Compared to metals or metal alloys conventionally used for manufacturingmicro-mechanical parts, such as toothed wheels, articulated parts orsprings, silicon has the advantage of having a density that is 3 to 4times lower and therefore of having a very reduced inertia and of beinginsensitive to magnetic fields. These advantages are particularlyinteresting in the horological field both for isochronism and theoperating duration of the timepiece when the energy source is formed ofa spring.

Silicon is however known to be sensitive to shocks, which may benecessary during assembly, inevitable in operation or accidental whenfor example the user knocks his wristwatch against something or dropsit.

EP patent No 1 422 436 discloses a silicon hairspring formed of a spiralshaped bar coated over its entire surface with a layer of amorphoussilicon oxide. According to this document, the first thermal coefficientof Young's modulus for silicon oxide is opposite to that of silicon.Thus, the combination of a core made of silicon with an external coatingof oxide is said to allow a reduction in said first thermal coefficient.

This prior art document does not mention the problem of shocksensitivity of parts made of silicon.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a solution that aimsto improve the mechanical resistance of a silicon micro-mechanical partand in particular its resistance to shocks.

Therefore the invention concerns a silicon micro-mechanical partaccording to one of independent claim 1 or 6.

The invention also concerns a method for manufacturing a reinforcedsilicon part according to claim 2. This method enables the formation, inparticular by thermal oxidation, the thick amorphous layer whichconsiderably increases the mechanical properties of said part as will beexplained in the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will appear moreclearly from the following description of an example embodiment, thisexample being given purely by way of non-limiting illustration withreference to the annexed drawings, in which:

FIG. 1 shows the initial cross-section of a silicon hairspring;

FIG. 2 corresponds to FIG. 1 after the deposition of an amorphousmaterial; and

FIG. 3 shows an additional step of deposition of an anti frictioncoating.

DETAILED DESCRIPTION OF THE INVENTION

A hairspring mounted in a horological movement the malfunction of whichis very easy to detect, simply by observing the movement stop if thehairspring happens to break, as will be explained hereinafter, has beentaken here by way of example.

The hairspring is obtained by known etching techniques from a siliconplate of slightly smaller thickness than the desired final height forthe hairspring.

One could for example use the reactive ionic etching technique (RIE) andgive the hairspring the shape which is considered most appropriate, asdisclosed for example in International Patent Application W02004/070476.

Given the very small dimensions of a hairspring, a batch of hairspringscan be manufactured in one time on the same plate.

FIG. 1 shows the cross-section of a hairspring having a core made ofsilicon, reference 3 designating the initial external surface. When thishairspring is left for a certain amount of time in the surrounding air,it naturally covers itself with silicon dioxide called “native oxide”(not shown) the thickness of which is substantially comprised between 1and 10 nm.

FIG. 2 shows the same cross-section of the hairspring after it has beentreated according to the invention, by a surface thermal oxidationbetween 900° C. and 1200° C. To this effect the protocol disclosed inthe publication “Semiconductors devices: physics and technology (ed.John Wiley & Sons, ISBN 0-471-87424-8, 01.01 1985, p.341-355) isapplied. Thus, it takes approximately 10 hours at a temperature of 1100°C. to obtain a thickness of silicon dioxide of about 1.9 micron. As canbe seen in FIG. 2 the silicon dioxide is formed using silicon, thesurface 3 of which moves backwards in order to create a new interface 5with the formed SiO₂. Conversely, given that SiO₂ has a lower density,the external surface 7 of SiO₂ extends beyond the initial surface of thehairspring. The positions of these separation lines 3, 5 , and 7 are notshown to scale, but it is obvious that knowledge of the physicalproperties of Si and SiO₂ and the thermal treatment characteristicsallows the initial dimensions to be calculated for etching thehairspring in order to have the desired dimensions at the end of thistreatment.

During a first series of tests, the mechanical resistance of nonoxidized silicon parts and oxidized silicon parts was tested from themanufacturing stage to the assembly stage.

During the manufacture of a batch of silicon parts, the parts need to bemanipulated at different manufacturing stages. For the specific casedescribed in this report, silicon parts originating from two siliconplates which have undergone identical steps are considered.

The parts have subsequently been mounted in a movement. During thetests, the parts are attached to a steel arbour and are pinched withtweezers and measurement setting. During final assembly on the movement,the center of the part is driven on to a solid arbour.

The following table summarizes the results of this test carried out on19 non oxidized parts and 36 oxidized parts.

Non oxidized parts Oxidized parts Plates Initial Remainder SuccessInitial Remainder Success 1 10 4 40% 16 16 100% 2 9 6 67% 20 20 100%Total 19 10 53% 36 36 100%During this test, comparison of the success rate of a complete chain ofoperations shows clearly that oxidized silicon parts are less fragilethan the same parts without oxidisation.

The mechanical properties of an ordinary silicon hairspring (FIG. 1) anda hairspring modified according to the invention (FIG. 2) have also beencompared in a real situation after assembly in the shock test using ashock pendulum of 5000 g.

Two identical movements, in which a non treated hairspring and ahairspring modified according to the invention have been mounted, havebeen subjected to this mechanical resistance test.

The movements fitted with the non oxidized hairspring or having a verythin deposition of native oxide stopped rapidly because of breakage ofthe hairsprings due to the shocks.

The movements fitted with the hairspring according to the inventionresisted the shocks for a long time and kept the working and isochronismthereof remained satisfactory for more than 30 weeks while being worn.

Thus, surprisingly, replacing one material, silicon, with a material oflower density, silicon dioxide, increases mechanical resistance, whileone might logically have expected a decrease in mechanical resistance.

In the example which has just been described, the “thick amorphouslayer” was silicon dioxide. In an equivalent manner this layer could beformed with other deposition methods, using other materials such assilicon nitride or carbide or titanium carbide or nitride.

This example shows that all external surfaces of the parts are uniformlycoated with a thick amorphous deposition. Of course the use ofappropriate masks allows deposition on only selected portions of thepart, i. e. on portions which are particularly mechanically stressed.Conversely, for example after a complete coating of SiO₂, it is possibleto eliminate certain portions of the coating by chemical etch with BHF,for example for esthetical reasons or for forming another type ofcoating.

FIG. 3 shows a variant wherein an additional step adds a coating 4 madeof a material selected for its tribological properties on the thickamorphous layer.

The foregoing description was made using a hairspring for a horologicalmovement by way of example, but it is obvious that the same advantageswould be found for any other parts of a watch movement (toothed wheel,escapement wheel, pallets, pivoted parts, etc. . . . ) and moregenerally any parts of a micro-mechanism without departing from thescope of the present invention.

1-10. (canceled)
 11. A silicon micromechanical part intended to beintegrated in an horological mechanism, said part being selected fromthe group comprising toothed wheels, escapement wheels, pallets, pivotedparts, and passive parts, wherein said part is coated over its entiresurface with silicon dioxide and wherein the thickness of the saidcoating is five times greater than the thickness of the native silicondioxide.
 12. A method for manufacturing a reinforced siliconmicromechanical part, said part being selected from the group comprisingtoothed wheels, escapement wheels, pallets, pivoted parts, and passiveparts, said method including, the successive steps consisting in:etching said part or a batch of said parts in a silicon plate,depositing over the entire surface of said part, in one or severalsteps, a silicon oxide layer, said deposition being made by thermaloxidation, at a temperature ranging from 900° C. to 1200° C., of thesurface of said part(s) for a sufficient period of time to obtain asilicon dioxide layer having a thickness which is at least five timesgreater than the thickness of native silicon dioxide.
 13. The methodaccording to claim 12, wherein, in the first step of the method, thepart is etched with slightly smaller dimensions than the desired finaldimensions.
 14. The method according to claim 12, wherein it furthercomprises an additional step consisting in coating at least partiallythe silicon dioxide deposition with a coating of a material selected forits tribological properties, such as diamond like carbon or carbonnanotubes.
 15. The method according to claim 12, wherein, after the stepof depositing the silicon dioxide layer, it comprises a step ofeliminating certain portions of said layer by chemical etching with BHF.16. Silicon micromechanical part intended to be integrated in anhorological mechanism, wherein it is obtained by the method according toclaim
 12. 17. The part according to claim 16, wherein the silicondioxide has a thickness greater than 50 nm.
 18. The part according toclaim 11, wherein the silicon dioxide has a thickness greater than 50nm.
 19. The part according to claim 11, wherein for the portions ofsilicon dioxide in contact with other parts of a kinematic chain, thesilicon dioxide is also at least partially coated with a coatingselected for its tribological properties, such as diamond like carbon(DLC) or carbon nanotubes.
 20. Silicon micromechanical part intended tobe integrated in an horological mechanism, wherein it is obtained by themethod according to claim
 13. 21. Silicon micromechanical part intendedto be integrated in an horological mechanism, wherein it is obtained bythe method according to claim
 14. 22. Silicon micromechanical partintended to be integrated in an horological mechanism, wherein it isobtained by the method according to claim 15.